Chapter 13 - Analyzing Subnet Masks

Question 1

True or False. The subnet bits of a subnet mask is just the difference between the network bits and the prefix length.

Answer 1

True. If the prefix was /27 and it was a class C address (24 bits) there would be 3 subnet bits.

Question 2

What are the rules of any subnet mask denoted in binary?

Answer 2

• There must not be interleaving between 1s and 0s (E.g. 10101101 is not allowed but 11110000 is)
• If 1s exist they are on the left
• If 0s exist they are on the right

Question 3

What is a DDN subnet mask?

Answer 3

DDN stands for Dotted Decimal Notation and converts the 8 bit binary sequence into it’s decimal value. (e.g. 11111111.00000000.00000000.00000000 = 255.0.0.0).

Question 4

What is a prefix subnet mask?

Answer 4

A prefix subnet mask is denoted by a / with the number of binary 1s in the binary subnet mask after it. (e.g. 11111111.00000000.00000000.00000000 = /8). Another name for this is the slash mask or CIDR mask.

Question 5

True or False. Each octet of a binary subnet mask will start at 128.

Answer 5

True. This is because the leftmost binary value is 128. From there each consecutive 1 will add each binary position’s value to 128.
00000000 = 0
10000000 = 128
11000000 = 192
11100000 = 224
11110000 = 240
11111000 = 248
11111100 = 252
11111110 = 254
11111111 = 255

Question 6

What are some of the different uses of a subnet?

Answer 6

• To define the size of the prefix (combined network and subnet parts) part of the addresses in a subnet
• To define the size of the host part of the addresses in a subnet
• Can be used to calculate the number of hosts in a subnet
• Allows a network designer to communicate design details such as the number of subnet and host bits to the devices in a network
• Can be used to calculate the number of subnets in an entire classful network
• Can be used in binary calculations to find both the subnet ID (network address) and broadcast address

Question 7

What is Classless Addressing?

Answer 7

• The concept that an IPv4 address has two parts, the prefix part and the host part as defined by the subnet mask with no consideration for the class.
• As opposed to classful addressing which lists an IPv4 address as three parts and abides by class rules.

Question 8

What is Classful Addressing?

Answer 8

The concept that an IPv4 address has three parts, the network, subnet, and host parts as defined by the mask and Class rules. As opposed to classless addressing which lists an IPv4 address as two parts and does not abide by class rules.

Question 9

To Find the Subnet ID using Decimal

Answer 9

Do 256 - <interesting> (=magic number) and multiply the magic number (0 IS INCLUDED) until you reach the interesting octet of the IP or the nearest number to it. This is the subnet ID.</interesting>

Question 10

To Find the Broadcast Address using Decimal

Answer 10

Do 256 - <interesting> (=magic number) and add the magic number to the interesting octet of the subnet ID then take away 1. This is the broadcast address.</interesting>

Question 11

To summarise multiple subnets into a single larger subnet

Answer 11

Either:
- Convert each network into binary and find how many bits are common. Once the bits stop being
common this is the answer

Example:
172.16.0.0
10101100.00010000.00000000.00000000

172.23.0.0
10101100.00010111.00000000.00000000
The 13th bit is the last common bit so /13 is the answer.